Co-transmission of acetylcholine and GABA regulates hippocampal states

Vagus nerve stimulation (VNS) preventing postoperative cognitive dysfunction (POCD): two potential mechanisms in cognitive function

Postoperative cognitive dysfunction (POCD) impacts a significant number of patients annually, frequently impairing their cognitive abilities and resulting in unfavorable clinical outcomes. Aimed at addressing cognitive impairment, vagus nerve stimulation (VNS) is a therapeutic approach, which was used in many mental disordered diseases, through the modulation of vagus nerve activity. In POCD model, the enhancement of cognition function provided by VNS was shown, demonstrating VNS effect on cognition in POCD. In the present study, we primarily concentrates on elucidating the role of the VNS improving the cognitive function in POCD, via two potential mechanisms: the inflammatory microenvironment and epigenetics. This study provided a theoretical support for the feasibility that VNS can be a potential method to enhance cognition function in POCD.

Single-nucleus analysis reveals dysregulated oxidative phosphorylation in Down syndrome basal forebrain at birth

INTRODUCTION

Basal forebrain cholinergic neurons (BFCNs) are integral to learning, attention, and memory, and are prone to degeneration in Down syndrome (DS), Alzheimer's disease, and other neurodegenerative diseases. However, the mechanisms that lead to degeneration of these neurons are not known.

METHODS

Single-nuclei gene expression and ATAC sequencing were performed on postmortem human basal forebrain from unaffected control and DS tissue samples at 0-2 years of age (n=4 each).

RESULTS

Sequencing analysis of postmortem human basal forebrain identifies gene expression differences in early postnatal DS early in life. Genes encoding proteins associated with energy metabolism pathways, specifically oxidative phosphorylation and glycolysis, and genes encoding antioxidant enzymes are upregulated in DS BFCNs.

DISCUSSION

Multiomic analyses reveal that energy metabolism may be disrupted in DS BFCNs by birth. Increased oxidative phosphorylation and the accumulation of reactive oxygen species byproducts may be early contributors to DS BFCN neurodegeneration.

Cholinergic dynamics in the septo-hippocampal system provide phasic multiplexed signals for spatial novelty and correlate with behavioral states

In the hippocampal formation, cholinergic modulation from the medial septum/diagonal band of Broca (MSDB) is known to correlate with the speed of an animal's movements at sub-second timescales and also supports spatial memory formation. Yet, the extent to which sub-second cholinergic dynamics, if at all, align with transient behavioral and cognitive states supporting the encoding of novel spatial information remains unknown. In this study, we used fiber photometry to record the temporal dynamics in the population activity of septo-hippocampal cholinergic neurons at sub-second resolution during a hippocampus-dependent object location memory task using ChAT-Cre mice. Using a general linear model, we quantified the extent to which cholinergic dynamics were explained by changes in movement speed, behavioral states such as locomotion, grooming, and rearing, and hippocampus-dependent cognitive states such as recognizing a novel location of a familiar object. The data show that cholinergic dynamics contain a multiplexed code of fast and slow signals i) coding for the logarithm of movement speed at sub-second timescales, ii) providing a phasic spatial novelty signal during the brief periods of exploring a novel object location, and iii) coding for environmental novelty at a seconds-long timescale. Furthermore, behavioral event-related phasic cholinergic activity around the onset and offset of the behavior demonstrates that fast cholinergic transients help facilitate a switch in cognitive and behavioral state before and during the onset of behavior. These findings enhance understanding of the mechanisms by which cholinergic modulation contributes to the coding of movement speed and encoding of novel spatial information.

Basal forebrain innervation of the amygdala: an anatomical and computational exploration

Theta oscillations of the mammalian amygdala are associated with processing, encoding and retrieval of aversive memories. In the hippocampus, the power of the network theta oscillation is modulated by basal forebrain (BF) GABAergic projections. Here, we combine anatomical and computational approaches to investigate if similar BF projections to the amygdaloid complex provide an analogous modulation of local network activity. We used retrograde tracing with fluorescent immunohistochemistry to identify cholinergic and non-cholinergic parvalbumin- or calbindin-immunoreactive BF neuronal subgroups targeting the input (lateral and basolateral nuclei) and output (central nucleus and the central bed nucleus of the stria terminalis) regions of the amygdaloid complex. We observed a dense non-cholinergic, putative GABAergic projection from the ventral pallidum (VP) and the substantia innominata (SI) to the basolateral amygdala (BLA). The VP/SI axonal projections to the BLA were confirmed using viral anterograde tracing and transsynaptic labeling. We tested the potential function of this VP/SI-BLA pathway in a 1000-cell biophysically realistic network model, which incorporated principal neurons and three major interneuron groups of the BLA, together with extrinsic glutamatergic, cholinergic, and VP/SI GABAergic inputs. We observed in silico that theta-modulation of VP/SI GABAergic projections enhanced theta oscillations in the BLA via their selective innervation of the parvalbumin-expressing local interneurons. Ablation of parvalbumin-, but not somatostatin- or calretinin-expressing, interneurons reduced theta power in the BLA model. These results suggest that long-range BF GABAergic projections may modulate network activity at their target regions through the formation of a common interneuron-type and oscillatory phase-specific disinhibitory motif.

Deletion of Neuroligins from Astrocytes Does Not Detectably Alter Synapse Numbers or Astrocyte Cytoarchitecture by Maturity

Astrocytes perform multifarious roles in the formation, regulation, and function of synapses in the brain, but the mechanisms involved are incompletely understood. Interestingly, astrocytes abundantly express neuroligins, postsynaptic adhesion molecules that function as synaptic organizers by binding to presynaptic neurexins. Here we examined the function of neuroligins in astrocytes with a rigorous genetic approach that uses the conditional deletion of all major neuroligins (Nlgn1-3) in astrocytes in vivo and complemented this approach by a genetic deletion of neuroligins in glia cells that are co-cultured with human neurons. Our results show that early postnatal deletion of neuroligins from astrocytes in vivo has no detectable effect on cortical or hippocampal synapses and does not alter the cytoarchitecture of astrocytes when evaluated in young adult mice. Moreover, deletion of astrocytic neuroligins in co-cultures of human neurons produced no detectable consequences for the formation and function of synapses. Thus, astrocytic neuroligins are unlikely to fundamentally shape synapse formation or astrocyte morphogenesis but likely perform other important roles that remain to be discovered.

Distinct release properties of glutamate/GABA co-transmission serve as a frequency-dependent filtering of supramammillary inputs

Glutamate and GABA co-transmitting neurons exist in several brain regions; however, the mechanism by which these two neurotransmitters are co-released from the same synaptic terminals remains unclear. Here, we show that the supramammillary nucleus (SuM) to dentate granule cell synapses, which co-release glutamate and GABA, exhibit differences between glutamate and GABA release properties in paired-pulse ratio, Ca2+-sensitivity, presynaptic receptor modulation, and Ca2+ channel-vesicle coupling configuration. Moreover, uniquantal synaptic responses show independent glutamatergic and GABAergic responses. Morphological analysis reveals that most SuM terminals form distinct glutamatergic and GABAergic synapses in proximity, each characterized by GluN1 and GABAAα1 labeling, respectively. Notably, glutamate/GABA co-transmission exhibits distinct short-term plasticities, with frequency-dependent depression of glutamate and frequency-independent stable depression of GABA. Our findings suggest that glutamate and GABA are co-released from different synaptic vesicles within the SuM terminals, and reveal that distinct transmission modes of glutamate/GABA co-release serve as frequency-dependent filters of SuM inputs.

GABA Receptors and Kv7 Channels as Targets for GABAergic Regulation of Acetylcholine Release in Frog Neuromuscular Junction

Effects of gamma-aminobutyric acid (GABA) and some selective GABAergic ligands on the quantal acetylcholine (ACh) release in the frog neuromuscular contacts were investigated using combination of microelectrode technique with fluorescent and immunohistochemical assays. Significant attenuation of ACh release was observed in the presence of GABA as well as selective GABAA and GABAB receptor agonists. Neither GABAA nor GABAB antagonists abolished to full extent this effect of GABA. Fluorescent assay allowed to detect the GABA-induced opening of K+ channels, which was inhibited by XE-991, a selective antagonist of Kv7 type. Electrophysiological recordings of endplate potentials in the presence of XE-991 confirmed the contribution of Kv7 type potassium channels to the effects of GABA on ACh release that was not associated with GABAA and GABAB receptors activation. Note that XE-991 effectively precluded the action of retigabine, neuronal Kv7 channel opener, on ACh release. Immunohistochemical assay revealed that frog mature skeletal muscle fibers contain a significant amount of GABA, and substantial amount of GABA can be released in the extracellular space at the muscle contractions induced by prolonged high-frequency nerve stimulation. Besides, some binding sites for exogenous GABA were detected on the plasma membranes. It is concluded that GABA, in addition to affecting GABAA and GABAB receptors, can directly activate Kv7 channels, thereby negatively modulating the evoked ACh release. Endogenous GABA may serve as a retrograde regulator of neurotransmitter exocytosis.

Neonatal anesthesia with remimazolam Reduces the expression of synaptic proteins and increases depressive behavior in adult mice

The demand for pediatric anesthesia has risen in decades, raising concerns about the neurotoxic potential of anesthetics like remimazolam, which may impact neurodevelopment and later cognitive function. This study utilized a neonatal mouse model to assess remimazolam's neurodevelopmental effects. Results indicate that remimazolam-exposed mice displayed cognitive impairment and depressive behaviors in adulthood. Acute reductions in synaptic protein expression post-anesthesia were observed, along with long-term decreases in hippocampal choline acetyltransferase levels, reduced dendritic spine density in the CA1 region, and microglial proliferation. Collectively, these findings suggest that remimazolam can induce neurotoxicity and neuroinflammation, leading to synaptic dysfunction and associated cognitive and behavioral deficits.

Cellular and circuit architecture of the lateral septum for reward processing

The lateral septum (LS) is composed of heterogeneous cell types that are important for various motivated behaviors. However, the transcriptional profiles, spatial arrangement, function, and connectivity of these cell types have not been systematically studied. Using single-nucleus RNA sequencing, we delineated diverse genetically defined cell types in the LS that play distinct roles in reward processing. Notably, we found that estrogen receptor 1 (Esr1)-expressing neurons in the ventral LS (LSEsr1) are key drivers of reward seeking via projections to the ventral tegmental area, and these neurons play an essential role in methamphetamine (METH) reward and METH-seeking behavior. Extended exposure to METH increases the excitability of LSEsr1 neurons by upregulating hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, thereby contributing to METH-induced locomotor sensitization. These insights not only elucidate the intricate molecular, circuit, and functional architecture of the septal region in reward processing but also reveal a neural pathway critical for METH reward and behavioral sensitization.

Role of adaptor protein complexes in generating functionally distinct synaptic vesicle pools

The synaptic vesicle (SV) cycle ensures the release of neurotransmitters and the replenishment of SVs to sustain neuronal activity. Multiple endocytosis and sorting pathways contribute to the recapture of the SV membrane and proteins after fusion. Adaptor protein (AP) complexes are among the critical components of the SV retrieval machinery. The canonical clathrin adaptor AP2 ensures the replenishment of most SVs across many neuronal populations. An alternative AP1/AP3-dependent process mediates the formation of a subset of SVs that differ from AP2 vesicles in molecular composition and respond preferentially during higher frequency firing. Furthermore, recent studies show that vesicular transporters for different neurotransmitters depend to a different extent on the AP3 pathway and this affects the release properties of the respective neurotransmitters. This review focuses on the current understanding of the AP-dependent molecular and functional diversity among SVs. We also discuss the contribution of these pathways to the regulation of neurotransmitter release across neuronal populations.

Functionally linked amygdala and prefrontal cortical regions are innervated by both single and double projecting cholinergic neurons

Cholinergic cells have been proposed to innervate simultaneously those cortical areas that are mutually interconnected with each other. To test this hypothesis, we investigated the cholinergic innervation of functionally linked amygdala and prefrontal cortical regions. First, using tracing experiments, we determined that cholinergic cells located in distinct basal forebrain (BF) areas projected to the different nuclei of the basolateral amygdala (BLA). Specifically, cholinergic cells in the ventral pallidum/substantia innominata (VP/SI) innervated the basal nucleus (BA), while the horizontal limb of the diagonal band of Broca (HDB) projected to its basomedial nucleus (BMA). In addition, cholinergic neurons in these two BF areas gave rise to overlapping innervation in the medial prefrontal cortex (mPFC), yet their axons segregated in the dorsal and ventral regions of the PFC. Using retrograde-anterograde viral tracing, we demonstrated that a portion of mPFC-projecting cholinergic neurons also innervated the BLA, especially the BA. By injecting retrograde tracers into the mPFC and BA, we found that 28% of retrogradely labeled cholinergic cells were double labeled, which typically located in the VP/SI. In addition, we found that vesicular glutamate transporter type 3 (VGLUT3)-expressing neurons within the VP/SI were also cholinergic and projected to the mPFC and BA, implicating that a part of the cholinergic afferents may release glutamate. In contrast, we uncovered that GABA is unlikely to be a co-transmitter molecule in HDB and VP/SI cholinergic neurons in adult mice. The dual innervation strategy, i.e., the existence of cholinergic cell populations with single as well as simultaneous projections to the BLA and mPFC, provides the possibility for both synchronous and independent control of the operation in these cortical areas, a structural arrangement that may maximize computational support for functionally linked regions. The presence of VGLUT3 in a portion of cholinergic afferents suggests more complex functional effects of cholinergic system in cortical structures.

Neuroanatomical Substrates of Circuit-Specific Cholinergic Modulation across the Primate Anterior Cingulate Cortex

Acetylcholine is a robust neuromodulator of the limbic system and a critical regulator of arousal and emotions. The anterior cingulate cortex (ACC) and the amygdala (AMY) are key limbic structures that are both densely innervated by cholinergic afferents and interact with each other for emotional regulation. The ACC is composed of functionally distinct dorsal (A24), rostral (A32), and ventral (A25) areas that differ in their connections with the AMY. The structural substrates of cholinergic modulation of distinct ACC microcircuits and outputs to AMY are thought to depend on the laminar and subcellular localization of cholinergic receptors. The present study examines the distribution of muscarinic acetylcholine receptors, m1 and m2, on distinct excitatory and inhibitory neurons and on AMY-targeting projection neurons within ACC areas, via immunohistochemistry and injections of neural tracers into the basolateral AMY in adult rhesus monkeys of both sexes. We found that laminar densities of m1+ and m2+ expressing excitatory and inhibitory neurons depended on area and cell type. Among the ACC areas, ventral subgenual ACC A25 exhibited greater m2+ localization on presynaptic inhibitory axon terminals and greater density of m1+ and m2+ expressing AMY-targeting (tracer+) pyramidal neurons. These patterns suggest robust cholinergic disinhibition and potentiation of amygdalar outputs from the limbic ventral ACC, which may be linked to the hyperexcitability of this subgenual ACC area in depression. These findings reveal the anatomical substrate of diverse cholinergic modulation of specific ACC microcircuits and amygdalar outputs that mediate cognitive-emotional integration and dysfunctions underlying stress and affective disorders.

Hippocampal replay sequence governed by spontaneous brain-wide dynamics

Neurons in the hippocampus exhibit spontaneous spiking activity during rest that appears to recapitulate previously experienced events. While this replay activity is frequently linked to memory consolidation and learning, the underlying mechanisms are not well understood. Recent large-scale neural recordings in mice have demonstrated that resting-state spontaneous activity is expressed as quasi-periodic cascades of spiking activity that pervade the forebrain, with each cascade engaging a high proportion of recorded neurons. Hippocampal ripples are known to be coordinated with cortical dynamics; however, less is known about the occurrence of replay activity relative to other brain-wide spontaneous events. Here we analyzed responses across the mouse brain to multiple viewings of natural movies, as well as subsequent patterns of neural activity during rest. We found that hippocampal neurons showed time-selectivity, with individual neurons responding consistently during particular moments of the movie. During rest, the population of time-selective hippocampal neurons showed both forward and time-reversed replay activity that matched the sequence observed in the movie. Importantly, these replay events were strongly time-locked to brain-wide spiking cascades, with forward and time-reversed replay activity associated with distinct cascade types. Thus, intrinsic hippocampal replay activity is temporally structured according to large-scale spontaneous physiology affecting areas throughout the forebrain. These findings shed light on the coordination between hippocampal and cortical circuits thought to be critical for memory consolidation.

Cholinergic Control of GnRH Neuron Physiology and Luteinizing Hormone Secretion in Male Mice: Involvement of ACh/GABA Cotransmission

Gonadotropin-releasing hormone (GnRH)-synthesizing neurons orchestrate reproduction centrally. Early studies have proposed the contribution of acetylcholine (ACh) to hypothalamic control of reproduction, although the causal mechanisms have not been clarified. Here, we report that in vivo pharmacogenetic activation of the cholinergic system increased the secretion of luteinizing hormone (LH) in orchidectomized mice. 3DISCO immunocytochemistry and electron microscopy revealed the innervation of GnRH neurons by cholinergic axons. Retrograde viral labeling initiated from GnRH-Cre neurons identified the medial septum and the diagonal band of Broca as exclusive sites of origin for cholinergic afferents of GnRH neurons. In acute brain slices, ACh and carbachol evoked a biphasic effect on the firing rate in GnRH neurons, first increasing and then diminishing it. In the presence of tetrodotoxin, carbachol induced an inward current, followed by a decline in the frequency of miniature postsynaptic currents (mPSCs), indicating a direct influence on GnRH cells. RT-PCR and whole-cell patch-clamp studies revealed that GnRH neurons expressed both nicotinic (α4β2, α3β4, and α7) and muscarinic (M1-M5) AChRs. The nicotinic AChRs contributed to the nicotine-elicited inward current and the rise in firing rate. Muscarine via M1 and M3 receptors increased, while via M2 and M4 reduced the frequency of both mPSCs and firing. Optogenetic activation of channelrhodopsin-2-tagged cholinergic axons modified GnRH neuronal activity and evoked cotransmission of ACh and GABA from a subpopulation of boutons. These findings confirm that the central cholinergic system regulates GnRH neurons and activates the pituitary-gonadal axis via ACh and ACh/GABA neurotransmissions in male mice.

Synaptic and dendritic architecture of different types of hippocampal somatostatin interneurons

GABAergic inhibitory neurons fundamentally shape the activity and plasticity of cortical circuits. A major subset of these neurons contains somatostatin (SOM); these cells play crucial roles in neuroplasticity, learning, and memory in many brain areas including the hippocampus, and are implicated in several neuropsychiatric diseases and neurodegenerative disorders. Two main types of SOM-containing cells in area CA1 of the hippocampus are oriens-lacunosum-moleculare (OLM) cells and hippocampo-septal (HS) cells. These cell types show many similarities in their soma-dendritic architecture, but they have different axonal targets, display different activity patterns in vivo, and are thought to have distinct network functions. However, a complete understanding of the functional roles of these interneurons requires a precise description of their intrinsic computational properties and their synaptic interactions. In the current study we generated, analyzed, and make available several key data sets that enable a quantitative comparison of various anatomical and physiological properties of OLM and HS cells in mouse. The data set includes detailed scanning electron microscopy (SEM)-based 3D reconstructions of OLM and HS cells along with their excitatory and inhibitory synaptic inputs. Combining this core data set with other anatomical data, patch-clamp electrophysiology, and compartmental modeling, we examined the precise morphological structure, inputs, outputs, and basic physiological properties of these cells. Our results highlight key differences between OLM and HS cells, particularly regarding the density and distribution of their synaptic inputs and mitochondria. For example, we estimated that an OLM cell receives about 8,400, whereas an HS cell about 15,600 synaptic inputs, about 16% of which are GABAergic. Our data and models provide insight into the possible basis of the different functionality of OLM and HS cell types and supply essential information for more detailed functional models of these neurons and the hippocampal network.

Increased excitability of dentate gyrus mossy cells occurs early in life in the Tg2576 model of Alzheimer's disease

INTRODUCTION

Hyperexcitability in Alzheimer's disease (AD) emerge early and contribute to disease progression. The dentate gyrus (DG) is implicated in hyperexcitability in AD. We hypothesized that mossy cells (MCs), regulators of DG excitability, contribute to early hyperexcitability in AD. Indeed, MCs generate hyperexcitability in epilepsy.

METHODS

Using the Tg2576 model and WT mice (∼1month-old), we compared MCs electrophysiologically, assessed c-Fos activity marker, Aβ expression and mice performance in a hippocampal-dependent memory task.

RESULTS

Tg2576 MCs exhibit increased spontaneous excitatory events and decreased inhibitory currents, increasing the charge transfer excitation/inhibition ratio. Tg2576 MC intrinsic excitability was enhanced, and showed higher c-Fos, intracellular Aβ expression, and axon sprouting. Granule cells only showed changes in synaptic properties, without intrinsic changes. The effects occurred before a memory task is affected.

DISCUSSION

Early electrophysiological and morphological alterations in Tg2576 MCs are consistent with enhanced excitability, suggesting an early role in DG hyperexcitability and AD pathophysiology.

HIGHLIGHTS

∘ MCs from 1 month-old Tg2576 mice had increased spontaneous excitatory synaptic input. ∘ Tg2576 MCs had reduced spontaneous inhibitory synaptic input. ∘ Several intrinsic properties were abnormal in Tg2576 MCs. ∘ Tg2576 GCs had enhanced synaptic excitation but no changes in intrinsic properties. ∘ Tg2576 MCs exhibited high c-Fos expression, soluble Aβ and axonal sprouting.

Stimulation of the muscarinic receptor M4 regulates neural precursor cell proliferation and promotes adult hippocampal neurogenesis

Cholinergic signaling plays a crucial role in the regulation of adult hippocampal neurogenesis; however, the mechanisms by which acetylcholine mediates neurogenic effects are not completely understood. Here, we report the expression of muscarinic acetylcholine receptor subtype M4 (M4 mAChR) on a subpopulation of neural precursor cells (NPCs) in the adult mouse hippocampus, and demonstrate that its pharmacological stimulation promotes their proliferation, thereby enhancing the production of new neurons in vivo. Using a targeted ablation approach, we also show that medial septum (MS) and the diagonal band of Broca (DBB) cholinergic neurons support both the survival and morphological maturation of adult-born neurons in the mouse hippocampus. Although the systemic administration of an M4-selective allosteric potentiator fails to fully rescue the MS/DBB cholinergic lesion-induced decrease in hippocampal neurogenesis, it further exacerbates the impairment in the morphological maturation of adult-born neurons. Collectively, these findings reveal stage-specific roles of M4 mAChRs in regulating adult hippocampal neurogenesis, uncoupling their positive role in enhancing the production of new neurons from the M4-induced inhibition of their morphological maturation, at least in the context of cholinergic signaling dysfunction.

Simultaneous detection of neurotransmitters and Cu2+ using double-bore carbon fiber microelectrodes via fast-scan cyclic voltammetry

There is a great demand to broaden our understanding of the multifactorial complex etiology of neurodegenerative diseases to aid the development of more efficient therapeutics and slow down the progression of neuronal cell death. The role of co-transmission and the effect of environmental factors on such diseases have yet to be explored adequately, mainly due to the lack of a proper analytical tool that can perform simultaneous multi-analyte detection in real time with excellent analytical parameters. In this study, we report a simple fabrication protocol of a double-bore carbon-fiber microelectrode (CFM) capable of performing rapid simultaneous detection of neurotransmitters and Cu2+via fast-scan cyclic voltammetry (FSCV) in Tris buffer. After imaging our CFMs via optical microscopy and scanning electron microscopy to ensure the intact nature of the two electrodes in our electrode composite, we performed a detailed analysis of the performance characteristics of our double-bore CFM in five different analyte mixtures, Cu2+-5HT, Cu2+-DA, Cu2+-AA, 5-HT-DA, and 5-HT-AA in Tris buffer, by applying different analyte-specific FSCV waveforms simultaneously. Calibration curves for each analyte in each mixture were plotted while extracting the analytical parameters such as the limit of detection (LOD), linear range, and sensitivity. We also carried out a control experiment series for the same mixtures with single-bore CFMs by applying one waveform at a time to compare the capabilities of our double-bore CFMs. Interestingly, except for the Cu2+-DA solution, all other combinations showed improved LOD, linear ranges, and sensitivity when detecting simultaneously with double-bore CFMs compared to single-bore CFMs, an excellent finding for developing this sensor for future in vivo applications.

Plastic rearrangement of basal forebrain parvalbumin-immunoreactive neurons in the kainite model of epilepsy

Temporal lobe epilepsy (TLE) is the most prevalent form of epilepsy, through the neuronal mechanisms of this syndrome remain elusive. In addition to the temporal lobe structures, it was found that the basal forebrain cholinergic cells are also involved in epileptogenesis. However, little is known about the involvement of the basal forebrain GABAergic neurons in epilepsy; despite this, they largely project to the temporal lobe and are crucial for the regulation of the hippocampal circuitry. In this study, we assessed epilepsy-induced changes in parvalbumin (PARV) immunoreactive neurons of the medial septum (MS) and of the magnocellular preoptic nucleus (MCPO) using the kainic acid (KA) model in rats. In addition, we estimated the respective changes in the cholinergic varicosities in the MS, where we observed a significant reduction in the PARV cell number (12849 ± 2715 vs. 9372 ± 1336, p = .029) and density (16.2 ± 2.62 vs. 10.5 ± 1.00 per .001 mm3, p =.001), and an increase in the density of cholinergic varicosities (47.9 ± 11.1 vs. 69.4 ± 17.8 per 30,000 µm2, p =.036) in KA-treated animals. In the MCPO, these animals showed a significant increase in somatic volume (827.9 ± 235.2 µm3 vs. 469.9 ± 79.6 µm3, p = .012) and total cell number (2268.6 ± 707.1 vs. 1362.4 ± 262.0, p =.028). These results show that the basal forebrain GABAergic cell populations undergo numerical and morphological changes in epileptic animals, which may contribute to an increased vulnerability of brain circuits to epilepsy and epilepsy-related functional impairments.

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. In vivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

Emerging computational motifs: Lessons from the retina

The retinal neuronal circuit is the first stage of visual processing in the central nervous system. The efforts of scientists over the last few decades indicate that the retina is not merely an array of photosensitive cells, but also a processor that performs various computations. Within a thickness of only ∼200 µm, the retina consists of diverse forms of neuronal circuits, each of which encodes different visual features. Since the discovery of direction-selective cells by Horace Barlow and Richard Hill, the mechanisms that generate direction selectivity in the retina have remained a fascinating research topic. This review provides an overview of recent advances in our understanding of direction-selectivity circuits. Beyond the conventional wisdom of direction selectivity, emerging findings indicate that the retina utilizes complicated and sophisticated mechanisms in which excitatory and inhibitory pathways are involved in the efficient encoding of motion information. As will become evident, the discovery of computational motifs in the retina facilitates an understanding of how sensory systems establish feature selectivity.

Synaptic and circuit functions of multitransmitter neurons in the mammalian brain

Neurons in the mammalian brain are not limited to releasing a single neurotransmitter but often release multiple neurotransmitters onto postsynaptic cells. Here, we review recent findings of multitransmitter neurons found throughout the mammalian central nervous system. We highlight recent technological innovations that have made the identification of new multitransmitter neurons and the study of their synaptic properties possible. We also focus on mechanisms and molecular constituents required for neurotransmitter corelease at the axon terminal and synaptic vesicle, as well as some possible functions of multitransmitter neurons in diverse brain circuits. We expect that these approaches will lead to new insights into the mechanism and function of multitransmitter neurons, their role in circuits, and their contribution to normal and pathological brain function.

The neurobiology and therapeutic potential of multi-targeting β-secretase, glycogen synthase kinase 3β and acetylcholinesterase in Alzheimer's disease

Alzheimer's Disease (AD) is the most common form of dementia, affecting almost 50 million of people around the world, characterized by a complex and age-related progressive pathology with projections to duplicate its incidence by the end of 2050. AD pathology has two major hallmarks, the amyloid beta (Aβ) peptides accumulation and tau hyperphosphorylation, alongside with several sub pathologies including neuroinflammation, oxidative stress, loss of neurogenesis and synaptic dysfunction. In recent years, extensive research pointed out several therapeutic targets which have shown promising effects on modifying the course of the disease in preclinical models of AD but with substantial failure when transposed to clinic trials, suggesting that modulating just an isolated feature of the pathology might not be sufficient to improve brain function and enhance cognition. In line with this, there is a growing consensus that an ideal disease modifying drug should address more than one feature of the pathology. Considering these evidence, β-secretase (BACE1), Glycogen synthase kinase 3β (GSK-3β) and acetylcholinesterase (AChE) has emerged as interesting therapeutic targets. BACE1 is the rate-limiting step in the Aβ production, GSK-3β is considered the main kinase responsible for Tau hyperphosphorylation, and AChE play an important role in modulating memory formation and learning. However, the effects underlying the modulation of these enzymes are not limited by its primarily functions, showing interesting effects in a wide range of impaired events secondary to AD pathology. In this sense, this review will summarize the involvement of BACE1, GSK-3β and AChE on synaptic function, neuroplasticity, neuroinflammation and oxidative stress. Additionally, we will present and discuss new perspectives on the modulation of these pathways on AD pathology and future directions on the development of drugs that concomitantly target these enzymes.

Linking temporal coordination of hippocampal activity to memory function

Oscillations in neural activity are widespread throughout the brain and can be observed at the population level through the local field potential. These rhythmic patterns are associated with cycles of excitability and are thought to coordinate networks of neurons, in turn facilitating effective communication both within local circuits and across brain regions. In the hippocampus, theta rhythms (4-12 Hz) could contribute to several key physiological mechanisms including long-range synchrony, plasticity, and at the behavioral scale, support memory encoding and retrieval. While neurons in the hippocampus appear to be temporally coordinated by theta oscillations, they also tend to fire in sequences that are developmentally preconfigured. Although loss of theta rhythmicity impairs memory, these sequences of spatiotemporal representations persist in conditions of altered hippocampal oscillations. The focus of this review is to disentangle the relative contribution of hippocampal oscillations from single-neuron activity in learning and memory. We first review cellular, anatomical, and physiological mechanisms underlying the generation and maintenance of hippocampal rhythms and how they contribute to memory function. We propose candidate hypotheses for how septohippocampal oscillations could support memory function while not contributing directly to hippocampal sequences. In particular, we explore how theta rhythms could coordinate the integration of upstream signals in the hippocampus to form future decisions, the relevance of such integration to downstream regions, as well as setting the stage for behavioral timescale synaptic plasticity. Finally, we leverage stimulation-based treatment in Alzheimer's disease conditions as an opportunity to assess the sufficiency of hippocampal oscillations for memory function.

A brain cytokine-independent switch in cortical activity marks the onset of sickness behavior triggered by acute peripheral inflammation

Systemic inflammation triggers protective as well as pro-inflammatory responses in the brain based on neuronal and/or cytokine signaling, and it associates with acutely and protractedly disrupted cognition. However, the multiple mechanisms underlying the peripheral-central inflammatory signaling are still not fully characterized. We used intraperitoneal (i.p.) injection of lipopolysaccharide (LPS) in freely moving mice with chronically implanted electrodes for recording of local field potentials (LFP) and electrocorticography (ECoG) in the hippocampus and neocortex, respectively. We show here that a sudden switch in the mode of network activity occurred in both areas starting at 10-15 min after the LPS injection, simultaneously with a robust change from exploration to sickness behavior. This switch in cortical mode commenced before any elevations in pro-inflammatory cytokines IL-1β, TNFα, CCL2 or IL-6 were detected in brain tissue. Thereafter, this mode dominated cortical activity for the recording period of 3 h, except for a partial and transient recovery around 40 min post-LPS. These effects were closely paralleled by changes in ECoG spectral entropy. Continuous recordings for up to 72 h showed a protracted attenuation in hippocampal activity, while neocortical activity recovered after 48 h. The acute sickness behavior recovered by 72 h post-LPS. Notably, urethane (1.3 mg/kg) administered prior to LPS blocked the early effect of LPS on cortical activity. However, experiments under urethane anesthesia which were started 24 h post-LPS (with neuroinflammation fully developed before application of urethane) showed that both theta-supratheta and fast gamma CA1 activity were reduced, DG delta activity was increased, and sharp-wave ripples were abolished. Finally, we observed that experimental compensation of inflammation-induced hypothermia 24-48 h post-LPS promoted seizures and status epilepticus; and that LPS decreased the threshold of kainate-provoked seizures beyond the duration of acute sickness behavior indicating post-acute inflammatory hyperexcitability. Taken together, the strikingly fast development and initial independence of brain cytokines of the LPS-induced cortical mode, its spectral characteristics and simultaneity in hippocampus and neocortex, as well as inhibition by pre-applied urethane, strongly suggest that the underlying mechanisms are based on activation of the afferent vagus nerve and its mainly cholinergic ascending projections to higher brain areas.

Spectroscopic Determination of Acetylcholine (ACh): A Representative Review

Acetylcholine (ACh) is one of the most crucial neurotransmitters of the cholinergic system found in vertebrates and invertebrates and is responsible for many processes in living organisms. Disturbances in ACh transmission are closely related to dementia in Alzheimer's and Parkinson's disease. ACh in biological samples is most often determined using chromatographic techniques, radioenzymatic assays, enzyme-linked immunosorbent assay (ELISA), or potentiometric methods. An alternative way to detect and determine acetylcholine is applying spectroscopic techniques, due to low limits of detection and quantification, which is not possible with the methods mentioned above. In this review article, we described a detailed overview of different spectroscopic methods used to determine ACh with a collection of validation parameters as a perspective tool for routine analysis, especially in basic research on animal models on central nervous system. In addition, there is a discussion of examples of other biological materials from clinical and preclinical studies to give the whole spectrum of spectroscopic methods application. Descriptions of the developed chemical sensors, as well as the use of flow technology, were also presented. It is worth emphasizing the inclusion in the article of multi-component analysis referring to other neurotransmitters, as well as the description of the tested biological samples and extraction procedures. The motivation to use spectroscopic techniques to conduct this type of analysis and future perspectives in this field are briefly discussed.

Spinal cord injury leads to more neurodegeneration in the hippocampus of aged male rats compared to young rats

Although the disruptive effects of spinal cord injury (SCI) on the hippocampus have been confirmed in some animal studies, no study has investigated its retrograde manifestations in the hippocampus of aged subjects. Herein, we compared the aged rats with young ones 3 weeks after the induction of SCI (Groups: Sham.Young, SCI.Young, Sham.Aged, SCI.Aged). The locomotion, hippocampal apoptosis, hippocampal rhythms (Delta, Theta, Beta, Gamma) max frequency (Max.rf) and power, hippocampal neurogenesis, and hippocampal receptors (NMDA, GABA A, Muscarinic1/M1), which are important in the generation of rhythms and neurogenesis, were compared in aged rats in contrast to young rats. At the end of the third week, the number of apoptotic (Tunel⁺) cells in the hippocampus (CA1, DG) of SCI animals was significantly higher compared to the sham animals, and also, it was significantly higher in the SCI.Aged group compared to SCI.Young group. Moreover, the rate of neurogenesis (DCX⁺, BrdU⁺ cells) and expression of M1 and NMDA receptors were significantly lower in the SCI.Aged group compared to SCI.Young group. The power and Max.fr of all rhythms were significantly lower in SCI groups compared to sham groups. Despite the decrease in the power of rhythms in the SCI.Aged group compared to SCI.Young group, there was no significant difference between them, and in terms of Max.fr index, only the Max.fr of theta and beta rhythms were significantly lower in the SCI.Aged group compared to SCI.Young group. This study showed that SCI could cause more neurodegeneration in the hippocampus of aged animals compared to young animals.

Basal forebrain cholinergic signalling: development, connectivity and roles in cognition

Acetylcholine plays an essential role in fundamental aspects of cognition. Studies that have mapped the activity and functional connectivity of cholinergic neurons have shown that the axons of basal forebrain cholinergic neurons innervate the pallium with far more topographical and functional organization than was historically appreciated. Together with the results of studies using new probes that allow release of acetylcholine to be detected with high spatial and temporal resolution, these findings have implicated cholinergic networks in 'binding' diverse behaviours that contribute to cognition. Here, we review recent findings on the developmental origins, connectivity and function of cholinergic neurons, and explore the participation of cholinergic signalling in the encoding of cognition-related behaviours.

Age-dependent effects of estradiol on temporal memory: A role for the type 1 cannabinoid receptor?

This study investigated in male mice how age modulates the effects of acute 17β-estradiol (E2) on dorsal CA1 (dCA1)-dependent retention of temporal associations, which are critical for declarative memory. E2 was systemically injected to young (3-4 months old) and aged (22-24 months old) adult mice either (i) 1 h before the acquisition of an auditory trace fear conditioning (TFC) procedure allowing the assessment of temporal memory retention 24 h later or (ii) during in vivo electrophysiological recordings of CA3 to dCA1 synaptic efficacy under anesthesia. In young mice, E2 induced parallel dose-dependent reductions in memory and synaptic efficacy, i.e. an impairment in TFC retention and a long-term (NMDA receptor-dependent) depression of dCA1 synaptic efficacy as assessed by field excitatory postsynaptic potentials. In contrast, E2 tended to improved TFC retention whilst failing to change synaptic efficacy in aged mice. Age-dependent effects of E2 treatment were confirmed by immunohistochemical analyses of TFC acquisition-elicited dCA1 Fos activation. Thus, such an activation was respectively reduced and enhanced in young and aged E2-treated mice, compared to vehicle treatments. Hippocampal mRNA expression of estrogen receptors by RT-PCR analyses revealed an age-related increase in each receptor mRNA expression. In keeping with the key role of the endocannabinoid system in memory processes and CA3 to dCA1 synaptic plasticity, we next examined the role of cannabinoid type 1 receptors (CB₁-R) in the aforementioned age-dependent effects of E2. Having confirmed that mRNA expression of CB₁-R diminishes with age, we then observed that the deleterious effects of E2 on both memory and synaptic efficacy were both prevented by the CB₁-R antagonist Rimonabant whilst being absent in CB₁-R knock out mice. This study (i) reveals age-dependent effects of acute E2 on temporal memory and CA3 to dCA1 synaptic efficacy and (ii) suggests a key role of CB₁-R in mediating E2 deleterious effects in young adulthood. Aging-related reductions in CB₁-R might thus underlie E2 paradoxical effects across age.

Acute toxic effects of new synthetic cannabinoid on brain: Neurobehavioral and Histological: Preclinical studies

The psychoactive effects of new synthetic cannabinoids (SCs), MDMB-4en-PINACA, are being marketed as a blend of herbs and spices. This study aims to determine the behavioral, neurochemical, histopathological, and immunohistochemical alterations associated with the acute toxicity of MDMB-4en-PINACA compounds.

METHODS

Adult male albino rats were administered various toxic doses of the drug (1.5, 3, and 6 mg/kg), and behavioral studies were conducted 2 and 24 h later; animals were then sacrificed. Histopathological and neurochemical examinations were performed. Two hours after intraperitoneal.

RESULTS

Intraperitoneal injection of MDMB-4en-PINACA, horizontal movement, the number of stops, and mobility ratio were significantly impaired, along with coordination and balance. In addition, it led to a decline in spatial learning and memory, and neurotransmitter concentrations decreased significantly in a dose-dependent manner. Further examination of the cerebral cortex and hippocampus histopathology revealed pathological degeneration of small pyramidal cells.

CONCLUSION

Thus, these findings revealed that MDMB-4en-PINACA interferes with hippocampal function and impairs cognitive performance, highlighting the cognitive risk associated with SC abuse.

Developmental regulation of GABAergic gene expression in forebrain cholinergic neurons

Acetylcholine and GABA are often co-released, including from VIP-expressing neurons of the cortex, cortically-projecting neurons of the globus pallidus externus and basal forebrain, and hippocampal-projecting neurons of the medial septum. The co-release of the functionally antagonistic neurotransmitters GABA and acetylcholine (ACh) greatly expands the possible functional effects of cholinergic neurons and provides an additional exogenous source of inhibition to the cortex. Transgene expression suggests that nearly all forebrain cholinergic neurons in mice at some point in development express Slc32a1, which encodes the vesicular GABA transporter (VGAT). To determine the degree of co-expression of GABA and Ach handling proteins, we measured expression in adult mice of Slc32a1, Gad1 and Gad2 (which encode GAD67 and GAD65, respectively, the GABA synthetic enzymes) in cholinergic neurons using fluorescent in situ hybridization. We found that only a subset of cholinergic neurons express the necessary machinery for GABA release at a single time in adult mice. This suggests that GABA co-release from cholinergic neurons is dynamic and potentially developmentally regulated. By measuring expression of Slc32a1, Gad1, Gad2, and Chat in the basal forebrain and medial septum in mice from post-natal day 0 to 28, we noted abundant yet variable expressions of GABAergic markers across early development, which are subsequently downregulated in adulthood. This is in contrast with the forebrain-projecting pedunculopontine nucleus, which showed no evidence of co-expression of GABAergic genes. These results suggest that expression of GABA signaling machinery in the cortically-projecting cholinergic system peaks during early development before settling at a non-zero level that is maintained through adulthood.

Subcortical glutamatergic inputs exhibit a Hebbian form of long-term potentiation in the dentate gyrus

The hippocampus receives glutamatergic and GABAergic inputs from subcortical regions. Despite the important roles of these subcortical inputs in the regulation of hippocampal circuit, it has not been explored whether associative activation of the subcorticohippocampal pathway induces Hebbian plasticity of subcortical inputs. Here, we demonstrate that the hypothalamic supramammillary nucleus (SuM) to the dentate granule cell (GC) synapses, which co-release glutamate and GABA, undergo associative long-term potentiation (LTP) of glutamatergic, but not GABAergic, co-transmission. This LTP is induced by pairing of SuM inputs with GC spikes. We found that this Hebbian LTP is input-specific, requires NMDA receptors and CaMKII activation, and is expressed postsynaptically. By the net increase in excitatory drive of SuM inputs following LTP induction, associative inputs of SuM and the perforant path effectively discharge GCs. Our results highlight the important role of associative plasticity at SuM-GC synapses in the regulation of dentate gyrus activity and for the encoding of SuM-related information.

Loss of GABA co-transmission from cholinergic neurons impairs behaviors related to hippocampal, striatal, and medial prefrontal cortex functions

Introduction: Altered signaling or function of acetylcholine (ACh) has been reported in various neurological diseases, including Alzheimer's disease, Tourette syndrome, epilepsy among others. Many neurons that release ACh also co-transmit the neurotransmitter gamma-aminobutyrate (GABA) at synapses in the hippocampus, striatum, substantia nigra, and medial prefrontal cortex (mPFC). Although ACh transmission is crucial for higher brain functions such as learning and memory, the role of co-transmitted GABA from ACh neurons in brain function remains unknown. Thus, the overarching goal of this study was to investigate how a systemic loss of GABA co-transmission from ACh neurons affected the behavioral performance of mice. Methods: To do this, we used a conditional knock-out mouse of the vesicular GABA transporter (vGAT) crossed with the ChAT-Cre driver line to selectively ablate GABA co-transmission at ACh synapses. In a comprehensive series of standardized behavioral assays, we compared Cre-negative control mice with Cre-positive vGAT knock-out mice of both sexes. Results: Loss of GABA co-transmission from ACh neurons did not disrupt the animal's sociability, motor skills or sensation. However, in the absence of GABA co-transmission, we found significant alterations in social, spatial and fear memory as well as a reduced reliance on striatum-dependent response strategies in a T-maze. In addition, male conditional knockout (CKO) mice showed increased locomotion. Discussion: Taken together, the loss of GABA co-transmission leads to deficits in higher brain functions and behaviors. Therefore, we propose that ACh/GABA co-transmission modulates neural circuitry involved in the affected behaviors.

Molecularly defined and functionally distinct cholinergic subnetworks

Cholinergic neurons in the medial septum (MS) constitute a major source of cholinergic input to the forebrain and modulate diverse functions, including sensory processing, memory, and attention. Most studies to date have treated cholinergic neurons as a single population; as such, the organizational principles underling their functional diversity remain unknown. Here, we identified two subsets (D28K⁺ versus D28K^-) of cholinergic neurons that are topographically segregated in mice, Macaca fascicularis, and humans. These cholinergic subpopulations possess unique electrophysiological signatures, express mutually exclusive marker genes (kcnh1 and aifm3 versus cacna1h and gga3), and make differential connections with physiologically distinct neuronal classes in the hippocampus to form two structurally defined and functionally distinct circuits. Gain- and loss-of-function studies on these circuits revealed their differential roles in modulation of anxiety-like behavior and spatial memory. These results provide a molecular and circuitry-based theory for how cholinergic neurons contribute to their diverse behavioral functions.

Differential Subcellular Distribution and Release Dynamics of Cotransmitted Cholinergic and GABAergic Synaptic Inputs Modify Dopaminergic Neuronal Excitability

We identified three types of monosynaptic cholinergic inputs spatially arranged onto medial substantia nigra dopaminergic neurons in male and female mice: cotransmitted acetylcholine (ACh)/GABA, GABA-only, and ACh only. There was a predominant GABA-only conductance along lateral dendrites and soma-centered ACh/GABA cotransmission. In response to repeated stimulation, the GABA conductance found on lateral dendrites decremented less than the proximally located GABA conductance, and was more effective at inhibiting action potentials. While soma-localized ACh/GABA cotransmission showed depression of the GABA component with repeated stimulation, ACh-mediated nicotinic responses were largely maintained. We investigated whether this differential change in inhibitory/excitatory inputs leads to altered neuronal excitability. We found that a depolarizing current or glutamate preceded by cotransmitted ACh/GABA was more effective in eliciting an action potential compared with current, glutamate, or ACh/GABA alone. This enhanced excitability was abolished with nicotinic receptor inhibitors, and modulated by T- and L-type calcium channels, thus establishing that activity of multiple classes of ion channels integrates to shape neuronal excitability.SIGNIFICANCE STATEMENT Our laboratory has previously discovered a population of substantia nigra dopaminegic neurons (DA) that receive cotransmitted ACh and GABA. This study used subcellular optogenetic stimulation of cholinergic presynaptic terminals to map the functional ACh and GABA synaptic inputs across the somatodendritic extent of substantia nigra DA neurons. We determined spatially clustered GABA-only inputs on the lateral dendrites while cotransmitted ACh and GABA clustered close to the soma. We have shown that the action of GABA and ACh in cotransmission spatially clustered near the soma play a critical role in enhancing glutamate-mediated neuronal excitability through the activation of T- and L-type voltage-gated calcium channels.

Basal Forebrain Cholinergic Innervation Induces Depression-Like Behaviors Through Ventral Subiculum Hyperactivation

Malfunction of the ventral subiculum (vSub), the main subregion controlling the output connections from the hippocampus, is associated with major depressive disorder (MDD). Although the vSub receives cholinergic innervation from the medial septum and diagonal band of Broca (MSDB), whether and how the MSDB-to-vSub cholinergic circuit is involved in MDD is elusive. Here, we found that chronic unpredictable mild stress (CUMS) induced depression-like behaviors with hyperactivation of vSub neurons, measured by c-fos staining and whole-cell patch-clamp recording. By retrograde and anterograde tracing, we confirmed the dense MSDB cholinergic innervation of the vSub. In addition, transient restraint stress in CUMS increased the level of ACh in the vSub. Furthermore, chemogenetic stimulation of this MSDB-vSub innervation in ChAT-Cre mice induced hyperactivation of vSub pyramidal neurons along with depression-like behaviors; and local infusion of atropine, a muscarinic receptor antagonist, into the vSub attenuated the depression-like behaviors induced by chemogenetic stimulation of this pathway and CUMS. Together, these findings suggest that activating the MSDB-vSub cholinergic pathway induces hyperactivation of vSub pyramidal neurons and depression-like behaviors, revealing a novel circuit underlying vSub pyramidal neuronal hyperactivation and its associated depression.

Cholinergic regulation of object recognition memory

Object recognition memory refers to a basic memory mechanism to identify and recall various features of objects. This memory has been investigated by numerous studies in human, primates and rodents to elucidate the neuropsychological underpinnings in mammalian memory, as well as provide the diagnosis of dementia in some neurological diseases, such as Alzheimer's disease and Parkinson's disease. Since Alzheimer's disease at the early stage is reported to be accompanied with cholinergic cell loss and impairment in recognition memory, the central cholinergic system has been studied to investigate the neural mechanism underlying recognition memory. Previous studies have suggested an important role of cholinergic neurons in the acquisition of some variants of object recognition memory in rodents. Cholinergic neurons in the medial septum and ventral diagonal band of Broca that project mainly to the hippocampus and parahippocampal area are related to recognition memory for object location. Cholinergic projections from the nucleus basalis magnocellularis innervating the entire cortex are associated with recognition memory for object identification. Especially, the brain regions that receive cholinergic projections, such as the perirhinal cortex and prefrontal cortex, are involved in recognition memory for object-in-place memory and object recency. In addition, experimental studies using rodent models for Alzheimer's disease have reported that neurodegeneration within the central cholinergic system causes a deficit in object recognition memory. Elucidating how various types of object recognition memory are regulated by distinct cholinergic cell groups is necessary to clarify the neuronal mechanism for recognition memory and the development of therapeutic treatments for dementia.

Medial septum: relevance for social memory

Animal species are named social when they develop the capability of complex behaviors based on interactions with conspecifics that include communication, aggression, mating and parental behavior, crucial for well-being and survival. The underpinning of such complex behaviors is social memory, namely the capacity to discriminate between familiar and novel individuals. The Medial Septum (MS), a region localized in the basal forebrain, is part of the brain network involved in social memory formation. MS receives several cortical and subcortical synaptic and neuromodulatory inputs that make it an important hub in processing social information relevant for social memory. Particular attention is paid to synaptic inputs that control both the MS and the CA2 region of the hippocampus, one of the major MS output, that has been causally linked to social memory. In this review article, we will provide an overview of local and long range connectivity that allows MS to integrate and process social information. Furthermore, we will summarize previous strategies used to determine how MS controls social memory in different animal species. Finally, we will discuss the impact of an altered MS signaling on social memory in animal models and patients affected by neurodevelopmental and neurodegenerative disorders, including autism and Alzheimer’s Disease.

Cortical excitability and plasticity in Alzheimer's disease and mild cognitive impairment: A systematic review and meta-analysis of transcranial magnetic stimulation studies

BACKGROUND

Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique. When stimulation is applied over the primary motor cortex and coupled with electromyography measures, TMS can probe functions of cortical excitability and plasticity in vivo. The purpose of this meta-analysis is to evaluate the utility of TMS-derived measures for differentiating patients with Alzheimer's disease (AD) and mild cognitive impairment (MCI) from cognitively normal older adults (CN).

METHODS

Databases searched included PubMed, Embase, APA PsycInfo, Medline, and CINAHL Plus from inception to July 2021.

RESULTS

Sixty-one studies with a total of 2728 participants (1454 patients with AD, 163 patients with MCI, and 1111 CN) were included. Patients with AD showed significantly higher cortical excitability, lower cortical inhibition, and impaired cortical plasticity compared to the CN cohorts. Patients with MCI exhibited increased cortical excitability and reduced plasticity compared to the CN cohort. Additionally, lower cognitive performance was significantly associated with higher cortical excitability and lower inhibition. No seizure events due to TMS were reported, and the mild adverse response rate is approximately 3/1000 (i.e., 9/2728).

CONCLUSIONS

Findings of our meta-analysis demonstrate the potential of using TMS-derived cortical excitability and plasticity measures as diagnostic biomarkers and therapeutic targets for AD and MCI.

Induction of synapse formation by de novo neurotransmitter synthesis

A vital question in neuroscience is how neurons align their postsynaptic structures with presynaptic release sites. Although synaptic adhesion proteins are known to contribute in this process, the role of neurotransmitters remains unclear. Here we inquire whether de novo biosynthesis and vesicular release of a noncanonical transmitter can facilitate the assembly of its corresponding postsynapses. We demonstrate that, in both stem cell-derived human neurons as well as in vivo mouse neurons of purely glutamatergic identity, ectopic expression of GABA-synthesis enzymes and vesicular transporters is sufficient to both produce GABA from ambient glutamate and transmit it from presynaptic terminals. This enables efficient accumulation and consistent activation of postsynaptic GABAA receptors, and generates fully functional GABAergic synapses that operate in parallel but independently of their glutamatergic counterparts. These findings suggest that presynaptic release of a neurotransmitter itself can signal the organization of relevant postsynaptic apparatus, which could be directly modified to reprogram the synapse identity of neurons.

Divergent properties and independent regulation of striatal dopamine and GABA co-transmission

Substantia nigra pars compacta (SNc) dopamine neurons play a key role in regulating the activity of striatal circuits within the basal ganglia. In addition to dopamine, these neurons release several other transmitters, including the major inhibitory neurotransmitter γ-aminobutyric acid (GABA). Both dopamine and GABA are loaded into SNc synaptic vesicles by the vesicular monoamine transporter 2 (VMAT2), and co-release of GABA provides strong inhibition to the striatum by directly inhibiting striatal medium spiny projection neurons (MSNs) through activation of GABAA receptors. Here, we found that despite both dopamine and GABA being co-packaged by VMAT2, the properties of transmission, including Ca2+ sensitivity, release probability, and requirement of active zone scaffolding proteins, differ between the two transmitters. Moreover, the extent by which presynaptic neuromodulators inhibit co-transmission also varied. Differences in modulation and the mechanisms controlling release allow for independent regulation of dopamine and GABA signals despite both being loaded via similar mechanisms.

Long-Range GABAergic Projections of Cortical Origin in Brain Function

The study of long-range GABAergic projections has traditionally been focused on those with subcortical origin. In the last few years, cortical GABAergic neurons have been shown to not only mediate local inhibition, but also extend long-range axons to remote cortical and subcortical areas. In this review, we delineate the different types of long-range GABAergic neurons (LRGNs) that have been reported to arise from the hippocampus and neocortex, paying attention to the anatomical and functional circuits they form to understand their role in behavior. Although cortical LRGNs are similar to their interneuron and subcortical counterparts, they comprise distinct populations that show specific patterns of cortico-cortical and cortico-fugal connectivity. Functionally, cortical LRGNs likely induce timed disinhibition in target regions to synchronize network activity. Thus, LRGNs are emerging as a new element of cortical output, acting in concert with long-range excitatory projections to shape brain function in health and disease.

TrkA-cholinergic signaling modulates fear encoding and extinction learning in PTSD-like behavior

Recent studies have suggested that the use of cognitive enhancers as adjuncts to exposure-based therapy in individuals suffering from post-traumatic stress disorder (PTSD) may be beneficial. Brain cholinergic signaling through basal forebrain projections to the hippocampus is an established pathway mediating fear response and cognitive flexibility. Here we employed a genetic strategy to enhance cholinergic activity through increased signaling of the NGF receptor TrkA. This strategy leads to increased levels of the marker of cholinergic activation, acetylcholine synthesizing enzyme choline acetyltransferase, in forebrain cholinergic regions and their projection areas such as the hippocampus. Mice with increased cholinergic activity do not display any neurobehavioral abnormalities except a selective attenuation of fear response and lower fear expression in extinction trials. Reduction in fear response is rescued by the GABA antagonist picrotoxin in mutant mice, and, in wild-type mice, is mimicked by the GABA agonist midazolam suggesting that GABA can modulate cholinergic functions on fear circuitries. Importantly, mutant mice also show a reduction in fear processing under stress conditions in a single prolonged stress (SPS) model of PTSD-like behavior, and augmentation of cholinergic signaling by the drug donepezil in wild-type mice promotes extinction learning in a similar SPS model of PTSD-like behavior. Donepezil is already in clinical use for the treatment of dementia suggesting a new translational application of this drug for improving exposure-based psychotherapy in PTSD patients.

Colocalization of different neurotransmitter transporters on synaptic vesicles is sparse except for VGLUT1 and ZnT3

Vesicular transporters (VTs) define the type of neurotransmitter that synaptic vesicles (SVs) store and release. While certain mammalian neurons release multiple transmitters, it is not clear whether the release occurs from the same or distinct vesicle pools at the synapse. Using quantitative single-vesicle imaging, we show that a vast majority of SVs in the rodent brain contain only one type of VT, indicating specificity for a single neurotransmitter. Interestingly, SVs containing dual transporters are highly diverse (27 types) but small in proportion (2% of all SVs), excluding the largest pool that carries VGLUT1 and ZnT3 (34%). Using VGLUT1-ZnT3 SVs, we demonstrate that the transporter colocalization influences the SV content and synaptic quantal size. Thus, the presence of diverse transporters on the same vesicle is bona fide, and depending on the VT types, this may act to regulate neurotransmitter type, content, and release in space and time.

Target-specific control of olfactory bulb periglomerular cells by GABAergic and cholinergic basal forebrain inputs

The olfactory bulb (OB), the first relay for odor processing in the brain, receives dense GABAergic and cholinergic long-range projections from basal forebrain (BF) nuclei that provide information about the internal state and behavioral context of the animal. However, the targets, impact, and dynamic of these afferents are still unclear. How BF synaptic inputs modulate activity in diverse subtypes of periglomerular (PG) interneurons using optogenetic stimulation and loose cell-attached or whole-cell patch-clamp recording in OB slices from adult mice were studied in this article. GABAergic BF inputs potently blocked PG cells firing except in a minority of calretinin-expressing cells in which GABA release elicited spiking. Parallel cholinergic projections excited a previously overlooked PG cell subtype via synaptic activation of M1 muscarinic receptors. Low-frequency stimulation of the cholinergic axons drove persistent firing in these PG cells, thereby increasing tonic inhibition in principal neurons. Taken together, these findings suggest that modality-specific BF inputs can orchestrate synaptic inhibition in OB glomeruli using multiple, potentially independent, inhibitory or excitatory target-specific pathways.

Untangling Peripheral Sympathetic Neurocircuits

The sympathetic nervous system plays a critical role in regulating many autonomic functions, including cardiac rhythm. The postganglionic neurons in the sympathetic chain ganglia are essential components that relay sympathetic signals to target tissues and disruption of their activity leads to poor health outcomes. Despite this importance, the neurocircuitry within sympathetic ganglia is poorly understood. Canonically, postganglionic sympathetic neurons are thought to simply be activated by monosynaptic inputs from preganglionic cholinergic neurons of the intermediolateral cell columns of the spinal cord. Early electrophysiological studies of sympathetic ganglia where the peripheral nerve trunks were electrically stimulated identified excitatory cholinergic synaptic events in addition to retrograde action potentials, leading some to speculate that excitatory collateral projections are present. However, this seemed unlikely since sympathetic postganglionic neurons were known to synthesize and release norepinephrine and expression of dual neurochemical phenotypes had not been well recognized. In vitro studies clearly established the capacity of cultured sympathetic neurons to express and release acetylcholine and norepinephrine throughout development and even in pathophysiological conditions. Given this insight, we believe that the canonical view of ganglionic transmission needs to be reevaluated and may provide a mechanistic understanding of autonomic imbalance in disease. Further studies likely will require genetic models manipulating neurochemical phenotypes within sympathetic ganglia to resolve the function of cholinergic collateral projections between postganglionic neurons. In this perspective article, we will discuss the evidence for collateral projections in sympathetic ganglia, determine if current laboratory techniques could address these questions, and discuss potential obstacles and caveats.

Right Anterior Theta Hypersynchrony as a Quantitative Measure Associated with Autistic Traits and K-Cl Cotransporter KCC2 Polymorphism

Our aim was to use theta coherence as a quantitative trait to investigate the relation of the polymorphisms in NKCC1 (rs3087889) and KCC2 (rs9074) channel protein genes to autistic traits (AQ) in neurotypicals. Coherence values for candidate connection regions were calculated from eyes-closed resting EEGs in two independent groups. Hypersynchrony within the right anterior region was related to AQ in both groups (p < 0.05), and variability in this hypersynchrony was related to the rs9074 polymorphism in the total group (p < 0.05). In conclusion, theta hypersynchrony within the right anterior region during eyes-closed rest can be considered a quantitative measure for autistic traits. Replicating our findings in two independent populations with different backgrounds strengthens the validity of the current study.

Lack of Hyperinhibition of Oriens Lacunosum-Moleculare Cells by Vasoactive Intestinal Peptide-Expressing Cells in a Model of Temporal Lobe Epilepsy

Temporal lobe epilepsy remains a common disorder with no cure and inadequate treatments, potentially because of an incomplete understanding of how seizures start. CA1 pyramidal cells and many inhibitory interneurons increase their firing rate in the seconds-minutes before a spontaneous seizure in epileptic rats. However, some interneurons fail to do so, including those identified as putative interneurons with somata in oriens and axons targeting lacunosum-moleculare (OLM cells). Somatostatin-containing cells, including OLM cells, are the primary target of inhibitory vasoactive intestinal polypeptide and calretinin-expressing (VIP/CR) bipolar interneuron-selective interneurons, type 3 (ISI-3). The objective of this study was to test the hypothesis that in epilepsy inhibition of OLM cells by ISI-3 is abnormally increased, potentially explaining the failure of OLM recruitment when needed most during the ramp up of activity preceding a seizure. Stereological quantification of VIP/CR cells in a model of temporal lobe epilepsy demonstrated that they survive in epileptic mice, despite a reduction in their somatostatin-expressing (Som) cell targets. Paired recordings of unitary IPSCs (uIPSCs) from ISI-3 to OLM cells did not show increased connection probability or increased connection strength, and failure rate was unchanged. When miniature postsynaptic currents in ISI-3 were compared, only mIPSC frequency was increased in epileptic hippocampi. Nevertheless, spontaneous and miniature postsynaptic potentials were unchanged in OLM cells of epileptic mice. These results are not consistent with the hypothesis of hyperinhibition from VIP/CR bipolar cells impeding recruitment of OLM cells in advance of a seizure.

Functional Implications of Neurotransmitter Segregation

Here, we present and discuss the characteristics and properties of neurotransmitter segregation, a subtype of neurotransmitter cotransmission. We review early evidence of segregation and discuss its properties, such as plasticity, while placing special emphasis on its probable functional implications, either in the central nervous system (CNS) or the autonomic nervous system. Neurotransmitter segregation is a process by which neurons separately route transmitters to independent and distant or to neighboring neuronal processes; it is a plastic phenomenon that changes according to synaptic transmission requirements and is regulated by target-derived signals. Distant neurotransmitter segregation in the CNS has been shown to be related to an autocrine/paracrine function of some neurotransmitters. In retinal amacrine cells, segregation of acetylcholine (ACh) and GABA, and glycine and glutamate to neighboring terminals has been related to the regulation of the firing rate of direction-selective ganglion cells. In the rat superior cervical ganglion, segregation of ACh and GABA to neighboring varicosities shows a heterogeneous regional distribution, which is correlated to a similar regional distribution in transmission strength. We propose that greater segregation of ACh and GABA produces less GABAergic inhibition, strengthening ganglionic transmission. Segregation of ACh and GABA varies in different physiopathological conditions; specifically, segregation increases in acute sympathetic hyperactivity that occurs in cold stress, does not vary in chronic hyperactivity that occurs in hypertension, and rises in early ages of normotensive and hypertensive rats. Given this, we propose that variations in the extent of transmitter segregation may contribute to the alteration of neural activity that occurs in some physiopathological conditions and with age.